This invention relates to an insulated-gate transistor signal input device formed on an insulating substrate such as an array substrate incorporated in a liquid-crystal display device.
A typical active matrix liquid-crystal display device includes an array substrate in which pixel electrodes are arrayed in a matrix form, a counter substrate in which a counter electrode is formed to face the pixel electrodes, and a liquid-crystal layer held between the array substrate and counter substrate. The pixel electrodes are connected to a liquid-crystal driving circuit placed outside the array substrate, via the switching elements which are formed on the array substrate together with the pixel electrodes.
The switching element is constituted by an insulated-gate transistor such as a thin-film transistor (TFT) which has an insulated gate and a semiconductor layer formed on the array substrate. In recent years, there is a case where the liquid-crystal driving circuit is formed on the array substrate. In this case, the liquid-crystal driving circuit is formed as an insulated-gate transistor circuit constituted by a group of thin-film transistors having the same structure as that of the switching element.
When the array substrate has been electrostatically charged in the process of manufacturing the liquid-crystal display device, the insulated-gate transistor tends to be damaged or destroyed due to a surge voltage applied thereto according to the electrostatic charge.
For example, Jpn. Pat. Appln. KOKAI Publication No. 6-51346 and Jpn. Pat. Appln. KOKAI Publication No. 9-80471 disclose a protection diode circuit disposed near an Outer Lead-Bonding (OLB) pad to protect insulated-gate transistors from destruction caused by electrostatic charge applied from the OLB pad.
FIG. 1 shows a conventional liquid-crystal driving circuit in which the protection diode circuit disposed near an OLB pad for inputting a clock signal. In the liquid-crystal driving circuit, the clock signal is supplied from an OLB pad 41 through a clock line LlN to a clock buffer 42, and then from the clock buffer 42 through a clock line L2N to a driver section DR. The clock buffer 42 has a series of inverters 41-1, 42-2, . . . , 42-n each constituted by p- and n-channel thin-film transistors, and performs a process of driving, in response to the clock signal, the total load capacitance of the clock line extending into the driver section DR.
A protection diode circuit DiN is constituted by a pair of p-channel thin-film transistors Tr1 and Tr2 whose current paths are connected in series between a power source line VDD and the clock line L1N and a pair of n-channel thin-film transistors Tr3 and Tr4 whose current paths are connected in series between a ground line GND and the clock line L1N. Each of the thin-film transistors Tr1, Tr2, Tr3 and Tr4 has a gate electrode connected to an end of the current path and constitute a diode reverse-biased by the power source voltage.
With the liquid-crystal driving circuit, the protection diode circuit D1N removes electrostatic charge applied from the OLB pad 41. Specifically, when the thin-film transistors Tr1 and Tr2 or thin-film transistors Tr3 and Tr4 are forward-biased by the electrostatic charge, they are turned on to remove the electrostatic charge to the power source line VDD or ground line GND. However, the clock line L2N is electrically separated from the clock line L1N by the gate insulating films of the thin-film transistors of the clock buffer 42, the protection diode circuit D1N cannot remove electrostatic charge applied to the clock line L2N. As a result, a thin-film transistor of the clock buffer 42 connected to the clock line L2N is damaged by the electrostatic charge. In this case, not only the reliability of the liquid-crystal driving circuit but also that of the liquid-crystal display device is impaired seriously. Furthermore, when the thin-film transistor of the clock buffer 42 is destroyed electrostatically, this leads to a decrease in the yield of the liquid-crystal display device.